Evidence for adiabatic polaron hopping in La0.65Ca0.35MnO3

We have measured the oxygen isotope effect on the electrical properties of La0.65Ca0.35MnO3 under hydrostatic pressure up to 1.9 GPa. Special attentio n is paid to the conductivity activation energy, E-a, above the insulator-m etal transition temperature, T-c. E-a is a universal linear function of T-c . The change of E-a by pressure and isotopic mass can be scaled to the chan ge of T-c. These results can be qualitatively understood in terms of the do uble exchange model and the adiabatic hopping of Jahn Teller polarons.